1. Field of the Invention
This invention relates to a lens barrel in which an auto focus (hereinafter referred to as AF) mode and a manual focus (hereinafter referred to as MF) mode can be selectively changed over to thereby accomplish the focusing of a photo-taking lens system (a focusing optical system).
2. Related Background Art
A camera provided with a focus adjusting mechanism by AF (auto focus) can accomplish photographing appropriately and reliably by simply being turned to an object. Such a camera does not require cumbersome focusing and therefore is excellent in snapshot capability operability, etc. However, it is sometimes the case even with such an AF type camera that a distance sensor does not work, depending on object conditions and the object distance cannot be measured. In such cases, it is regarded as necessary to design the camera such that the focus adjusting mechanism is operated by a manual operation and focusing by so-called MF (manual focus) can be accomplished. It has heretofore been conceived to use, for example, an electromagnetic clutch or the like as a changeover device for selectively changing over the focus adjusting mechanism of this kind to the AF mode or the MF mode in response to a mode changeover operating device.
When in the focus adjusting mechanism of the AF mode/MF mode changeover type as described above, changeover operation is to be performed from the MF mode to the AF mode or from the AF mode to the MF mode, if the electromagnetic clutch as described above is used as the changeover device, a practical problem as will hereinafter be described has arisen. When the power source voltage of the camera drops to bring about the so-called 0sec hold state in which a halfdepression timer by the half-depression of a release button does not work, the half-depression switch changes over from its ON state to its OFF state, whereby the supply of electric power to the various portions of the camera is cut off. As a result the restoration to the AF mode or to the MF mode by the above-described electromagnetic clutch cannot be completed, and this has given rise to the problem that a reliable mode changeover operation cannot be accomplished.
When the electromagnetic clutch is thus stopped in the course of the changeover operation and the mode changeover is incomplete, there arises the problem that the focus adjusting mechanism does not work and focusing cannot be accomplished. Particularly, when effecting photographing with the mode changeover effected from the MF mode and focusing effected automatically in the AF mode, there arises the problem that the reliability of focusing is inadequate.